Abstract. The Nilotic family, a group of languages spoken in East Africa allows a uniform subgrouping on the basis of Neogrammarian principles of shared innovations. Nevertheless, there is also evidence for wave-like innovations cutting across intragenetic boundaries. For example, the original contrast between implosive and plosive stops, only retained in three Nilotic languages synchronically, must have been lost independently in different subgroups. Interestingly, implosion has been retained as a distinctive feature in Nilotic languages bordering on other languages where this feature also occurs. This distributional fact suggests that language contact and bilingualism favoured not only the innovation but also the retention of certain phonetic properties of sound systems. A more speculative aspect of the present contribution concerns sound change in expressive words. There is some evidence that separate mechanisms may be operative in such words.